Aims: Our overall aim is to understand the role of alveoli in the initiation of lung inflammation. Previously, we reported that alveolar TNFalpha induces vectorial crosstalk between epithelial and endothelial cells of intact alveolo-capillary membranes, and that increases in the AEC cytosolic Ca2+ (cytCa2+) drive the crosstalk. Here, we will test the hypothesis that the crosstalk messenger is H2O2 originating from mitochondria or NAD(P)H oxidase of AEC. Our Specific Aims are to define alveolo-capillary signaling in the context of the regulation of cytCa2+ and H2O2 production in AEC by receptor-mediated (Specific Aims 1 and 2) and receptor-independent (Specific Aim 3) mechanisms. Procedures: We will use the isolated, blood-perfused lung preparation for mouse and rat. Alveoli will be loaded with specific fluorophores for the detection of cytosolic, mitochondrial and endosomal store Ca2+, and for ROS production. Concomitantly, capillaries will be loaded with similar dyes to determine crosstalk responses. Single AEC and EC in the alveolo-capillary region will be optically imaged in real-time by of wide-angle and confocal microscopy. For proinflammatory activation, alveoli capillaries will be challenged with TNFalpha, LPS, arachidonate, concentrated HCI and photolytic uncaging of Ca2+. EC responses will be determined in terms of cytCa2+ and ROS production and the recruitment of leukocytes. The hypotheses will also be tested in mice containing genetically defective in NAD(P)H oxidase (gp91phox(-/-)), and in cPLA2. Significance: Although pathological stimuli in the alveolus induce lung inflammation, the role of the alveolar epithelial barrier in generating this response is not understood. Our research will reveal the importance of different modes of Ca2+ mobilization in AEC, namely those that are receptor-mediated versus receptor-independent, in establishing the messenger that translates the alveolar response to proinflammatory activation of EC in capillaries. This research is novel and it will advance our understanding of fundamental mechanisms regulating lung inflammation.